The long-term objective of this laboratory is to experimentally study the mechanical mechanisms of cochlear functions. This application is to extend a novel set of measurements of cochlear mechanical responses using a scanning laser interferometer to address the following fundamental questions: (1) What are the longitudinal extent and the actual wavelength of the traveling wave in the sensitive cochlea? (2) What is the vibration pattern of the BM in the radial direction? (3) What is the space-frequency relationship of BM vibration? (4) Can a resonance mechanism be demonstrated experimentally in the sensitive cochlea? Specific aim one will measure the longitudinal extent and the wavelength of BM vibration in sensitive gerbil cochleae. The hypothesis is that at low and intermediate intensities, the longitudinal extent of the BM vibration and the wavelength of the cochlear traveling wave are much smaller than previously thought. The data from this experiment will provide quantified parameters for testing current theories on the location of the cochlear amplifier and the mechanism of cochlear nonlinearity. Specific aim two will measure the magnitude and phase of BM vibration as a function of the radial location. The hypothesis that there is a significant phase variation across the BM width will be tested. This experiment will verify the putative temporal relationship between the outer hair cell (OHC)-generated force and the passive BM vibration. Specific aim three will measure the longitudinal patterns of BM vibrations at different frequencies and intensities. The hypothesis that the sharp tuning is a frequency representation of the spatially restricted longitudinal pattern of BM vibration will be tested by comparing the spatial data to the tuning curve measured from a single BM location. Results from this experiment will be critical for testing the fundamental theory of cochlear scaling symmetry. Specific aim four will investigate the cochlear resonance mechanism. The hypothesis that the BM responds to local stimulation as a resonant system will be tested by measuring BM responses to a local photoacoustic stimulation. The information from this study will enhance clinical interpretation of audiological tests and suggest new approaches for artificial hearing for rehabilitating hearing loss.